A SCR (Selective Catalytic Reduction) catalyst is a catalyst which purifies nitrogen oxides (NOx) by reducing the nitrogen oxides with ammonia (NH3). Especially, the SCR catalyst has started to be used for diesel engines which emit a comparatively large amount of nitrogen oxides. The SCR catalyst is referred to as a “NOx selective reduction catalyst.”
Reducing nitrogen oxides by the SCR catalyst requires to supply ammonia which serves as a reduction agent for nitrogen oxides to the SCR catalyst. A conventional apparatus therefore supplies a water (urea-water, urea aqueous solution) containing urea (CO(NH2)2═H2N—CO—NH2) to the SCR catalyst, in place of ammonia. The urea changes into ammonia and carbon dioxides by hydrolysis in the urea-SCR catalyst. The ammonia thus obtained by the hydrolysis reduces the nitrogen oxides in the SCR catalyst (refer to Patent Literature 1). The system where the urea-water is supplied to the SCR catalyst may be referred to as a “Urea SCR system”, and the SCR catalyst to which the urea-water is supplied may be referred to as a “urea-SCR catalyst.”
Meanwhile, there has been developed an engine comprising a supercharger, a high-pressure EGR system, and a low-pressure EGR system. The high-pressure EGR system is an exhaust gas recirculation system in which a high pressure EGR pipe is provided, the high pressure EGR pipe communicating between a position upstream of a turbine in an exhaust passage and a position downstream of a compressor in an intake passage, and a high pressure exhaust gas (high pressure EGR gas) is recirculated through the high pressure EGR pipe to the intake passage. The low pressure EGR system is an exhaust gas recirculation system in which a low pressure EGR pipe is provided, the low pressure EGR pipe communicating between a position downstream of the turbine in the exhaust passage and a position upstream of the compressor in the intake passage, and a low pressure exhaust gas (low pressure EGR gas) is recirculated through the low pressure EGR pipe to the intake passage. A temperature of the high pressure EGR gas recirculated by the high pressure EGR system is high. Therefore, it is difficult to recirculate a large amount of the high pressure EGR gas when, for example, a load of the engine is high. In contrast, a temperature of the low pressure EGR gas recirculated by the low pressure EGR system is low. Therefore, a large amount of the low pressure EGR gas can be recirculated even when a large amount of the high pressure EGR gas can not be recirculated. Consequently, an amount of NOx discharged from the engine can be reduced (refer to Patent Literature 2).